Deprecate SortField.setMissingValue()

[romseygeek]

Relates to #15480

Placeholder values for SortFields are set via setMissingValue(), after construction, which means
that they are inherently mutable. This commit deprecates setMissingValue() and instead
adds constructor parameters for missing values.

[benwtrent]

Its weird that its mutable to begin with...seems like a mistake. I wonder if it was mutable because certain sort fields just don't support setting missing value?

I did a quick look through the code base and through Solr & ES, there weren't obvious places where it was actually treated as mutable. Generally the pattern was 'construct...some logic...immediately set the value once on the new instance'

[romseygeek]

I wonder if it was mutable because certain sort fields just don't support setting missing value?

TBH I think it was immutable because it was 2002 and that's how Java was written then. And as you say, it hasn't really been an issue with actual usage. Even now, it's only really a problem for index sorts, although I suppose there could be some weird concurrency issues if a SortField was shared across multiple threads.